fix(html): drop the blanket link target, and mark only external links

[andiwand]

🤖 Generated with Claude Code

Fixes #730. Rebased onto main now that #756 and #757 are merged.

Every view was written with <base target="_blank">, so every link was a new tab. A browser has one; an embedded web view does not, and unless the host implements window opening the tap does nothing at all. Worst on the archive listing, where the links are the whole point: a zip rendered as a table of its entries and none of them could be opened.

Why not a config

The blanket base threw away a distinction the renderer already has and the host doesn't — whether a link points back into the same service or out to the web. pdf_file.cpp was already paying to undo it:

// Internal `#pN` links must override the document's `<base
// target="_blank">` or they open a new copy instead of scrolling.
<< (link.internal ? " target=\"_self\"" : "")

So the fix encodes the distinction instead of asking the embedder to re-answer it. is_safe_uri's scan becomes uri_kind → relative / external / refused, and the target follows from it:

	markup	browser	embedded web view
relative (content.xml, #p2, other.html)	no target	navigates in place	navigates in place — the listing works with no host code
external (https:, mailto:, …)	target="_blank" rel="noopener noreferrer"	new tab, as before	raises createWebViewWith / onCreateWindow — the host's hand-off-to-system-browser hook
refused (javascript:, …)	no href (#756)	—	—

<base target> goes, and with it the PDF's _self workaround and HtmlWriter::write_header_target, which nothing else used. OpenDocument.ios can drop its current WKUIDelegate workaround and just open Safari there — internal links no longer reach the delegate at all.

Test

• a_link_that_is_navigable_keeps_its_href extended: external links carry the target, and #bookmark / other.html / a/b.html carry none.
• New no_view_declares_a_document_wide_link_target across the csv, xml, text and markdown writers.
• PdfFile.link_annotations_render_as_anchors and the two page-view link tests updated: #pN and page1.html name no target, the /URI action names _blank.
• Full suite green.

Reference output

1525 files — every view carried the base tag. I verified mechanically that after stripping <base target="_blank"/>, target="_blank" rel="noopener noreferrer" and target="_self" from both sides, zero files differ; the pins are advanced to the regenerated commits.